Tim Pawlenty has become the latest Republican presidential candidate to sign a pledge from the National Organization for Marriage (NOM), according to Politico. The group is dedicated to fighting same sex marriage and preventing same sex couples from adopting children.

Republican candidates Michele Bachmann, Rick Santorum and Mitt Romney have also signed the pledge, vowing to support a federal marriage amendment defining marriage as one man and one woman.

By signing NOM's marriage pledge, the candidates also vowed to protect the Defense of Marriage Act in court, appoint conservative judges and a conservative attorney general, create a presidential commission to investigate harassment of same sex marriage opponents and put same sex marriage up to a vote in D.C.

"Marriage is an issue with an unbroken string of victories that unites Republicans, and we're pleased and honored the leading candidates in the race for the GOP nomination have spoken up for marriage," Brian Brown, president of NOM, said in a statement. "We expect the voters of Iowa, New Hampshire, and South Carolina will continue to reward candidates who champion marriage."

Not surprisingly, the only gay Republican presidential candidate, Fred Karger, criticized those who had signed the pledge. He told The Advocate that it was inappropriate for any presidential candidate to sign a pledge, especially one from a group like NOM, which is under investigation by the state of Maine Ethics Commission for money laundering.

In 2010, the Southern Poverty Law Center included NOM in its list of anti-gay groups. That same year, the LGBT rights organizations Human Rights Campaign (HRC) and the Courage Campaign launched a website targeting the group, called "NOM Exposed."

"NOM has become the leading anti-equality force in this country seemingly overnight," Fred Sainz, vice president of communications for HRC, said in 2010. "They’ve thrived on voters knowing nothing about them or who fuels them."

The NOM pledge is similar to another pledge from The Family Leader, a Iowa-based Christian conservative group. Michele Bachmann and Rick Santorum were the only two presidential candidates to sign the Family Leader pledge, which drew large amounts of criticism.